Journal of Experimental Botany, Vol. 52, No. 361, pp. 1665-1671,
August 1, 2001
© 2001 Oxford University Press
Original Papers |
Translocation of amino acids in the xylem of apple (Malus domestica Borkh.) trees in spring as a consequence of both N remobilization and root uptake
1 Dipartimento di Colture Arboree, Università di Bologna, Via F. Re 6, 40126 Bologna, Italy
2 Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB15 8QH, UK
Received 21 November 2000; Accepted 18 April 2001
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Abstract |
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Nitrogen is remobilized from storage for the growth of Malus domestica leaves each spring. Seasonal patterns of N translocation in the xylem sap as a consequence of remobilization were determined in 2-year-old ‘Golden delicious’ trees grafted on M9 rootstocks. The trees were grown in sand culture and 15NH415NO3 at 10.4 atom% abundance supplied during August–September. The following year no further N was supplied and destructive harvests were taken during bud burst and leaf growth to determine the patterns of N remobilization together with the isolation of xylem sap for an analysis of their amino acid profiles and 15N enrichments by GC-MS. The concentration of amino acids in the xylem sap rose following bud burst, peaked at full bloom and then fell again during petal fall and fruit set. The peak in amino acid concentration corresponded with the period when the rate of N remobilization was the fastest. The majority of labelled N was recovered in Asn, Gln+Glu and Asp demonstrating that they were being translocated as a consequence of remobilization. In a second experiment, 8-year-old trees growing in an orchard were fertilized with N either in the autumn or spring. Xylem sap samples were collected in the spring and early summer and, by comparison with the amino acid profiles recovered in trees from both treatments, Asn was identified as the main compound translocated as a consequence of both remobilization and root uptake of N, although there was evidence that root uptake of N occurred later. The data are discussed in relation to quantifying the internal cycling of N in trees.
Key words: Malus domestica, asparagine, N remobilization, xylem sap.
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Introduction |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Deciduous trees store nitrogen (N) during winter, which is remobilized in the spring for leaf growth (Millard, 1996
). Malus domestica Borkh. (apple) trees can remobilize most of their stored N before there is a rapid root uptake from the soil (Millard and Neilsen, 1989), with root uptake becoming increasingly important as the season progresses (Tromp and Ovaa, 1969; Neilsen et al., 1997). The relative contribution of remobilized N and that taken up by the roots to growth can be affected by a number of factors, including soil fertility (Millard, 1996) and the timing of fertilizer applications (Weinbaum et al., 1984; Sanchez et al., 1992; Tagliavini et al., 1999). M. domestica trees use remobilized N for the growth of spur leaves first (providing up to 87% of their N) and about the time of full bloom start to remobilize N for shoot leaves, by which time net uptake provides a greater contribution (Neilsen et al., 1997).
Measuring the contribution of N storage and remobilization to tree growth is difficult and often involves the use of 15N-enriched (Millard, 1996) or depleted (Weinbaum et al., 1984) isotopes. However, the use of isotopes is usually limited to sand culture experiments with small trees, because field studies can not allow for uptake of native soil N at natural abundance, thereby underestimating the internal cycling of N (Millard, 1996). An alternative method to assess N remobilization by trees could be to quantify the N translocation pattern in the xylem. Several studies have shown a peak of N concentration in the xylem sap during bud burst and leaf growth, which has been attributed to remobilization (Ferguson et al., 1983; Glavac and Jockheim, 1993; Schneider et al., 1994). Using 15N to label storage pools, Millard et al. showed that remobilization in young Betula pendula trees led to a 10-fold increase in the concentration of N in the xylem sap, due predominantly to increases in citrulline and glutamine (Millard et al., 1998).
Young M. domestica trees were grown in sand culture supplied with 15N in autumn. In the second year of the experiment the trees received no further N and a series of destructive harvests were taken along with the collection of xylem sap in order to (1) determine the relative contribution of N taken up in the autumn to remobilization the following spring, and (2) establish the pattern of amino acid translocation in the xylem as a consequence of N remobilization. A second experiment used mature trees growing in an orchard which was supplied with fertilizer either in the autumn or the following spring, then subsequently recovered xylem saps to (3) determine if the form of N translocation due to remobilization is the same in large field-grown trees as in young potted trees, and (4) determine if the pattern of amino acid translocation differs if derived from remobilization or root uptake.
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Materials and methods |
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Trees grown in sand culture
Apple trees of cv. Golden delicious were grafted on M9 clonal rootstocks in 1995, lifted from the nursery when dormant in the winter 1997 and immediately planted in 0.04 m3 pots containing sand. Trees were grown outdoors with the pots covered to protect them against direct sunlight. Between March and September pots were drip irrigated at 1–2 d intervals to bring them to field capacity. During this period, a total of 13 nutrient applications were made (one every 2 weeks), on each occasion providing each plant with 1.0 g N, 0.3 g P, 0.7 g K, 1.0 g Ca, 0.1 g Mg, 0.2 g S, 1.5 mg B, 3.0 mg Mn, 15.0 mg Fe, and 0.6 mg of both Zn and Cu. For the first 10 applications, up to 31 July, the N applied was at natural abundance (unlabelled N). Thereafter a total of three nutrient applications were made, on 1 August, 12 August and 9 September, where the N was provided as 15NH415NO3 enriched to 10.4 atom% abundance (labelled N) and all other nutrients provided as described above.
In January 1998, trees were carefully removed from the pots and after washing the sand from the root system, planted into fresh sand in new pots and arranged outdoors in a completely randomized design. From March onwards the trees continued to receive drip irrigation, and nutrient additions every 2 weeks, except that no N was provided during 1998. Twelve destructive harvests of three replicate trees were carried out from bud burst (10 March, 1998) to 25 May, 1998, each time recording the stage of phenological development. At each harvest, three trees were randomly selected and removed from their pots. Trees were divided into developing vegetative (shoot and leaves) and reproductive (flowers and fruits) organs, fine and coarse roots, stem, branches, and twigs. All tree organs were dried, weighed and milled to pass through a 5 mm screen prior to 15N analysis. A Tracer Mat continuous flow mass spectrometer (Finningan MAT, Hemel Hempstead, UK) was used for determination of 15N and total N. The 15N enrichment was used to calculate the amount of labelled N taken up after 1 August in 1997 and remobilized in 1998 to new growth (Millard and Neilsen, 1989).
Xylem sap collection was carried out just before tree harvesting, between 09.00 h and 11.00 h. Two portions of the main axis (2-year-old stem) of 30 cm length were collected at a height of 130 and 170 cm above ground from selected trees. A few cm of bark were removed from the cut end to avoid phloem contamination. The stem portion was then immediately placed in a Schlolander pressure chamber so that the section of wood with bark removed protruded. Xylem sap was collected by microcapillary tubes using a pressure of 0.1 MPa. Preliminary tests on xylem saps extracted from extra trees checked for contamination by cellular components, by measuring the ATP content of the sap (Strehler and Totter, 1952). No detectable ATP was found when a pressure of 0.1 MPa was used to collect the sap. When a pressure of 0.2 MP was used, the ATP content of the sap was 4 µmol m-3, rising to 11 µmol m-3 with a pressure of 0.6 MPa being applied. These ATP concentrations represented a cytoplasmic contamination of less than 0.1% in both cases (Schneider et al., 1996). A pressure of 0.1 MPa was used thereafter for the extraction of xylem saps from the trees used in the experiments.
Xylem saps were stored at -80 °C until analysis of the concentration of 15N enrichment of their individual amino acids by gas chromatography linked to mass spectrometry (GC-MS). The free amino acids in the xylem saps and the internal standard (nor-valine) were converted into their t-buthyldimethylsilyl (t-BDMS) derivatives as described earlier (Millard et al., 1998). The analyses of derivatives were carried out using GC-MS in the single ion recording mode with a VG TRIO 1 quadrupole mass spectrometer linked to a Fisons Series 8000 gas chromatrograph fitted with a AS800 autosampler (Fisons Instruments UK, Crawley, Sussex). The GC-MS working conditions are the same described previously (Millard et al., 1998) and resulted in a precision of isotope ratio analysis of ±0.3 atom% excess. Values of amino acid concentrations and their 15N enrichments were calculated as described earlier (Millard et al., 1998), and used to calculate the amounts of both labelled and unlabelled amino acid N as described previously (Millard and Neilsen, 1989).
Field-grown trees
The second experiment was established in the summer of 1998 in an apple orchard of cv. Golden delicious grafted on M9 in 1990 with planting distances of 3.5x1.5 m. Soil was sub-alkaline and loamy and had not received any N supply in the previous two years. Fruit yield in 1998 was about 7.6 kg per tree, a value considered about average for this variety in the area. During the vegetative season of 1998 nitrogen fertilization was withdrawn until September. Four rows of trees were selected and within each row, three sets of two adjacent trees were randomly attributed to one of the three treatments. The first set of trees received a single application of 40 g N per tree as granular NH4NO3 on 16 September 1998 (designated Autumn N) to increase N uptake in autumn and so enhance storage during the winter and so increase N remobilization the following spring. A second set of trees received the same amount of N on 6 March 1999 to increase N availability for uptake in spring (Spring N); this application was performed 13 d before bud burst occurred. On both occasions, the fertilizer was placed in a strip of 1.0 m width along either side of the tree row and irrigation was provided to ensure the fertilizer reached the roots and N was provided at natural abundance. The third set of trees remained unfertilized as a control.
To determine the effects of N fertilization on autumn N uptake, a sample of 40 leaves was collected on 4 November 1998, before leaf abscission started, from the mid portion of the shoots. The samples were dried, milled and analysed for total N by the Kjeldahl method. To assess the impact of N supply on soil N availability for plant uptake, soil cores were collected at a depth 0–40 cm and water-soluble nitrate, nitrite and ammonium extracted in distilled water (1 : 2, w : v) for 1 h with stirring, followed by filtration. Determinations were made by reflectometry using the Meck RQflex and specific tests (ammonium test n. 1.16892; nitrate test n. 1.16971 and nitrite test n. 1.16973.0001). More than 200 mm rain occurred between December 1998 and February 1999. Bud burst in 1999 occurred on March 19 Julian Day (JD) 78, the date xylem sap extraction started. Sap extraction was carried out as described above from one terminal twig between 20 and 25 cm in length. Nine sets of extractions were carried out in total on JDs 78, 83, 90, 95, 100 (corresponding to fall bloom), 108, 117, 126, and 138. Soil temperature was monitored at 5 cm depth throughout the sampling period, and increased linerarly from 9.2 °C (JD 78) to 17.8 °C (JD 138). The soil temperature at full bloom between 100–120 JD was between 11–14 °C. The xylem saps were analysed for their amino acid concentration as described above.
Data analysis
Data for the remobilization of labelled and unlabelled N were fitted with curves using the best-fit transition functions of Table curve 2 d software (SPSS Inc.). The patterns of amino acid concentrations in xylem saps were established by using curve-fit peak functions. In each case curves with the highest r2 values were reported.
Data from field experiments were subjected to ANOVA to determine the significance of difference among the treatments. The experiment had a randomized block design with four replicates, and differences between treatments were assessed by LSD test at the 0.05 level of probability.
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Results |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Trees grown in sand culture
Because trees did not receive any additional N in 1998, both the labelled and unlabelled N recovered in the new growth was remobilized (Fig. 1
). In both cases the rate of N remobilization increased slowly from bud burst until about 90 JD (balloon stage) and then rapidly, thereafter. The trees took up some 1.7±0.4 g labelled N tree-1 of which 730±82 mg was subsequently remobilized, representing 43% of the total. The labelled N provided less than one-quarter of the amount of unlabelled N remobilized.
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The concentration of amino acids in the xylem sap started to rise following bud burst, peaked between 100–120 JD (at the time of full bloom) and then fell during petal fall and fruit set (Fig. 2
). The peak in amino acid concentrations corresponded with the period between 90–120 JD when the rate of N remobilization was fastest, as shown by the slope of the curves in Fig. 1
. The maximum mean concentration of N recovered in the xylem sap at any of the harvests was 517±58 µg g-1 sap, on the harvest taken at 106 JD. At this time more than 95% of the total N in the xylem sap was recovered in four amino acids, Asn, Asp and Gln+Glu (Table 1). By 140 JD, after remobilization had finished, the concentration of total N in the sap had fallen to 94±25 µg g-1, with the three amino acids accounting for 89% of the total N, although there had been no significant decrease in the concentration of Ser, Gly, Pro, Ile, Leu, and Phe (Table 1).
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Because Asp, Asn and Gln (including Glu) were the major nitrogenous constituents recovered in the xylem saps, the concentrations of both their labelled and unlabelled forms were followed from bud burst until after fruit set. The data for Asn (the predominant amino acid) are shown in Fig. 2
. The best peak functions fitted to the data were always Asymmetric Logistic curves. Generally, the peak of the unlabelled form was 4–5 times greater than that of the labelled one. For all amino acids the peak in the concentration of labelled N occurred sooner than that of the unlabelled N (Table 2) which is in agreement with the observation that remobilization of labelled N finished sooner than that of unlabelled N (Fig. 1). The concentration of Asn in the sap increased sharply after day 90, reached the peak quickly and then decreasing slowly afterwards (Fig. 2).
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Field experiment
The range of amino acids recovered from xylem saps collected from the field experiment was similar to those from the pot-grown trees, with Asn, Asp and Gln+Glu predominating. The concentration of Asn in the sap from control trees, which had received no fertilizer, rose to 90 µg g-1 at 90 JD and then decreased to about 50 µg g-1 until 109 JD (Fig. 3). Therefore, in trees with a low N status there was a peak in Asn concentration lasting only a few days, as had been found in the pot-grown trees (Fig. 2). When trees were fertilized in the autumn, however, the concentration of Asn peaked at 101 JD and was significantly (P<0.01) higher then those from control trees. This demonstrated that the fertilization had increased the N status of the trees, which was confirmed by the fact that autumn-fertilized trees had a significantly higher (P<0.01) concentration of N in their leaves than the 4 November 1998 sampling (1.67% N and 1.47% N for fertilized and unfertilized trees, respectively). The concentration of 
and 
extracted from the soil samples from underneath the fertilized and control trees on 4 November 1998 was similar and less than 1.0 µg g-1. During October there was unusually heavy rainfall of 120 mm, which probably leached out any fertilizer N which had not been taken up by the trees and from November to February rainfall was 110 mm. Therefore, it is likely that the increased concentration of Asn in the xylem sap of autumn-fertilized trees was due to remobilization and not to uptake of N by the roots the following year.
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), fertilizer in the spring (
) or no fertilizer (
). The data are the mean from four replicate trees, and within a harvest date means with the same superscript letter are not significantly different at P<0.05.When the trees were fertilized in the spring, prior to bud burst, the concentration of N recovered in Asn was significantly (P<0.01) greater than control trees between 101–109, then decreased until 120 JD, when they were still greater than the control values, but not significantly so (Fig. 3
). These differences must have been due to uptake of N by roots and not remobilization. Therefore, Asn was the main form of N transported both as a result of remobilization and of root uptake in the spring. The concentration of Asp and Gln+Glu in saps were not significantly affected by either autumn or spring fertilization (data not shown).
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Discussion |
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As shown by several authors (Cooper et al., 1972
; Neilsen et al., 1997; Tromp, 1983) M. domestica has a high potential of remobilizing the N from perennial tissues to support the new vegetative and reproductive growth in spring. Such remobilization is unaffected by the amount of N supplied in spring (Millard and Neilsen, 1989) and can account for almost the entire amount of N recovered in the flowers and in the spur leaves at the time of flowering (Neilsen et al., 1997). A significant portion of the N stored in apple has been found in the root system (Tromp, 1983), not recovered in specific storage proteins as those typical for bark (Sauter and Neumann, 1994), but as soluble amino acids (Tromp and Ovaa, 1976). Nitrogen applied at the end of September, for example, increased the amounts of arginine in roots, which accounted almost entirely for the increase of soluble N in the roots of fertilized apple trees as compared to unfertilized ones (Tromp, 1983). In the present study remobilization of labelled N was almost completed at JD 130 and the rate of N remobilization was most rapid around the time of full bloom, when a peak of N concentration was recovered in the xylem sap. Afterwards, the decrease of N concentration in the sap started before remobilization finished, which could be due to a dilution of the xylem sap as a consequence of faster transpiration concomitant with leaf growth.
Few amino acids were responsible for spring remobilization of N in both the pot and the field-grown trees. Asn accounted for more than half the total amino acid N in the sap, regardless of the timing of sampling, followed by Gln and Asp, with only negligible concentrations of other amino acids found during remobilization. With the GC-MS methods used in this study it was not possible to analyse the xylem saps for Arg which has been reported in the xylem sap of apple (Tromp and Ovaa, 1976) as well as of pear trees (Anderson et al., 1995). In order to check if Arg was a significant component of the xylem saps from the trees in the study, sap samples from each harvest date were bulked together and analysed by HPLC using o-phthalaldehyde derivitization, as described previously (Millard, 1986). The concentration of Arg recovered was 48 µg amino acid N g-1 xylem sap, compared with 137 µg N g-1 in Asn and 66 and 55 µg N g-1 sap in Asp and Gln, respectively. This suggests that, averaged over the whole sampling period, Arg accounted for less than 15% of the N translocated in the xylem sap. Various compounds have been reported as the main form of N translocated in the xylem sap of trees. Gln is the main compound in Prunus spp (Anderson et al., 1995; Youssefi et al., 2000), Populus (Schneider et al., 1944), Picea abies (Stoerner et al., 1997), and Pinus spp (Barnes, 1963; Plassard et al., 2000). Citrulline, which is the precursor of Arg but not formed as a degradation product of Arg after protein turnover (Thompson, 1980), has been reported as the predominant N compound in xylem saps from a range of tree species (Barnes, 1963; Sheldrake and Northcote, 1968; Millard et al., 1998). There are few reports of Arg translocation accounting for the majority of N in the xylem sap. Schmidt and Stewart found Arg as the main compound in xylem saps from a wide range of tree species from savanna woodlands and deciduous monsoon forests in northern Australia during the dry season (Schmidt and Stewart, 1998). However, there was a shift to mainly amide N (Gln) during the wet season, which was interpreted as trees translocating N from root uptake as amides and from remobilization as Arg (Schmidt and Stewart, 1998). In contrast, coniferous evergreen trees translocate predominantly amides (Stoerner et al., 1997; Plassard et al., 2000), despite their potential to accumulate large amounts of Arg in their tissues as a consequence of either N fertilization of the soil (Stoerner et al., 1997) or atmospheric N deposition (Näsholm et al., 1997).
Some deciduous trees take particular advantage from N taken up late in the season when primary shoot growth has stopped, which is preferentially stored in the perennial organs and remobilized the following season (Tagliavini et al., 1999). This study's data show that the peak in the remobilization of labelled N occurred a few days before unlabelled N. This suggests that N taken up in the autumn is preferentially remobilized compared with N assimilated in spring and summer, explaining why autumn fertilization increases the amount of N stored during the winter (Millard and Thomson, 1989).
Before remobilization of stored N finishes in spring, M. domestica trees start taking up N by their roots (Neilsen et al., 1997), which is mainly reduced and incorporated into amino acid in the roots (Grasmanis and Nicholas, 1967). All the N in the xylem saps from the pot experiment was derived from remobilization, because no N was supplied to the roots during the sampling period. However, in the field experiment the composition of the xylem saps also reflected N uptake by roots, particularly in the trees which had been fertilized in the spring. The balance between N translocation from root uptake or remobilization might be affected by soil temperature. Low root zone temperatures can inhibit root growth and metabolism (Ryyppo et al., 1998; Wan et al., 1999), N uptake (Iivonen et al., 1999) and result in a slower sap flux rate (Anderson and Brodbeck, 1989; De Lucia et al., 1991) in a range of tree species. However, the root temperature during the sampling period (9–17 °C) is unlikely to have limited N uptake by apple trees, as shown previously (Bath, 1983; Toselli et al., 1999). Data in this study show that Asn is translocated as a consequence of both N remobilization and also N taken up by the roots directly, as previously suggested (Hill-Cottingham and Cooper, 1970; Cooper et al., 1972; Tromp and Ovaa, 1976).
Millard et al. raised the possibility of measuring the remobilization of N in spring by analysing the pattern of N translocation in the spring (Millard et al., 1998). The development of such an approach would be of great benefit as it would allow quantification of the contribution of internal cycling of N to the growth of field-grown trees without using 15N-enriched fertilizers. Data from this study suggest that for M. domestica such an approach would be difficult due to the fact that the trees use the same few amino acids for translocating to the canopy both N remobilized from storage and that from root uptake. The exact pattern of Asn concentration in the sap and the date on which concentrations peaked, however, differed depending on whether N came from remobilization or directly from root uptake. The sand culture experiment indicated that if growth of apple in spring relies on remobilization of N taken up the previous year, but not on spring N supply, then the concentration of Asn peaks around full bloom and decreases sharply afterwards. When N is available for spring uptake, on the contrary, Asn concentration in the sap increases from blossom to peak later. Therefore, by following the pattern of N translocation in the xylem it might be possible to estimate the pattern of N remobilization in spring and the need for root uptake of N.
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Acknowledgments |
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We thank Alistair Smith and Maureen Procee for their advice and help with the GC-MS analyses and Bruno Marangoni for support and encouragement. This work was funded in part by the Scottish Executive Rural Affairs Department and in part by the ‘Consiglio Nazionale delle Ricerche–CNR’ Project NITCAR.
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Notes |
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3 To whom correspondence should be addressed. Fax: +44 1224 311556. E-mail: p.millard{at}mluri.sari.ac.uk
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References |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Anderson PC, Brodbeck BV. 1989. Temperature and temperature preconditioning on flux and chemical composition of xylem exudates from muscadine grapevines. Journal of the American Society of Horticultural Science 114, 440–444.
Andersen PC, Brodbeck BV, Mizell III RF. 1995. Diurnal variations in tension, osmolarity and the composition of nitrogen and carbon assimilated in xylem fluid of Prunus persica, vitis hybrid and Pyrus communis. Journal of the American Society of Horticultural Science 120, 600–606.
Bath KS. 1983. Nutrient inflows into apple roots. Plant and Soil 71, 371–380.[Web of Science]
Barnes RL. 1963. Organic nitrogen compounds in tree xylem sap. Forest Science 9, 98–102.
Cooper DR, Hill-Cottingham DG, Shorthill J. 1972. Gradients in the nitrogenous constituents of the sap extracted from apple shoots of different ages. Journal of Experimental Botany 23, 247–254.
De Lucia EH, Day TA, Oquist G. 1991. The potential for photoinhibition of Pinus sylvestris L. seedlings exposed to high light and low soil temperature. Journal of Experimental Botany 42, 611–617.
Ferguson AR, Eiseman JA, Leonard JA. 1983. Xylem sap from Actinidia chinensis: seasonal changes in composition. Annals of Botany 51, 823–833.
Glavac V, Jockheim H. 1993. A contribution to understanding the internal nitrogen budget of beech (Fagus sylvatica L.). Trees 7, 237–241.
Grasmanis VO, Nicholas DJD. 1967. A nitrate reductase from apple roots. Phytochemistry 6, 217–218.[Web of Science]
Hill-Cottingham DG, Cooper DR. 1970. Effect of time of application of fertilizer nitrogen on the distribution and identity of the nitrogenous constituents of young apple trees. Journal of the Science of Food and Agriculture 21, 172–177.[Web of Science]
Iivonen S, Rikala R, Ryyppo A, Vapaavuori E. 1999. Responses of Scots pine (Pinus sylvestris) seedlings grown in different nutrient regimes to changing root zone temperature in spring. Tree Physiology 19, 951–958.
Millard P. 1986. The nitrogen content of potato (Solanum tuberosum L.) tubers in relation to nitrogen application—the effect on amino acid composition and yields. Journal of the Science of Food and Agriculture 37, 107–114.
Millard P. 1996. Ecophysiology of the internal cycling of nitrogen for tree growth. Journal of Plant Nutrition and Soil Science 159, 1–10.
Millard P, Neilsen GH. 1989. The influence of nitrogen supply on the uptake and remobilization of stored N for the seasonal growth of apple trees. Annals of Botany 63, 301–309.
Millard P, Thomson CM. 1989. The effect of the autumn senescence of leaves on the internal cycling of nitrogen for the spring growth of apple trees. Journal of Experimental Botany 40, 1285–1289.
Millard P, Wendler R, Hepburn A, Smith A. 1998. Variations in the amino acid composition of xylem sap of Betula pendula Roth. trees due to remobilization of stored N in the spring. Plant, Cell and Environment 21, 715–722.
Näsholm T, Nordin A, Edfast A-B, Hogberg P. 1997. Identification of coniferous forestry in the incipient nitrogen saturation through analysis of arginine and nitrogen-15 abundance of trees. Journal of Environmental Quality 26, 302–309.
Neilsen D, Millard P, Neilsen GH, Hogue EJ. 1997. Sources of N for leaf growth in a high-density apple (Malus domestica) orchard irrigated with ammonium nitrate solution. Tree Physiology 17, 733–739.
Plassard C, Bonafos B, Touraine B. 2000. Differential effects of mineral and organic N sources, and of ectomycorrhizal infection by Hebeloma cylindrosporanum, on growth and N utilization in Pinus pinaster. Plant, Cell and Environment 23, 1195–1205.
Ryyppo A, Iivonen S, Rikala R, Sutinen ML, Vapaavuori E. 1998. Responses of Scots pine seedlings to low root zone temperature in spring. Physiologia Plantarum 102, 503–512.
Sanchez EE, Righetti TL, Sugar D, Lombard PD. 1992. Effect of timing of nitrogen application on nitrogen partitioning between vegetative, reproductive and structural components of manure ‘Comice’ pears. Journal of Horticultural Sciences 67, 51–58.
Sauter JJ, Neumann V. 1994. The accumulation of storage materials in ray cells of poplar wood (Populusxcanadensis) (robusta): effect of ringing and defoliation. Journal of Plant Physiology 143, 21–26.[Web of Science]
Schmidt S, Stewart GR. 1998. Transport, storage and mobilization of nitrogen by trees and shrubs in the wet/dry tropics of northern Australia. Tree Physiology 18, 403–410.
Schneider A, Kreazwieser J, Schums R, Sauter JJ, Rennenberg H. 1994. Thiol and amino acid composition of the xylem sap of poplar trees. Canadian Journal of Botany 72, 347–351.
Schneider S, Gessler A, Weber P, von Sengbusch D, Hanemann U, Rennenberg H. 1996. Soluble N compounds in trees exposed to high loads of N: a comparison of spruce (Picea abies) and beech (Fagus sylvatica) grown under field conditions. New Phytologist 134, 103–114.[Web of Science]
Sheldrake AR, Northcote DH. 1968. Some constituents of xylem sap and their possible relationship to xylem differentiation. Journal of Experimental Botany 19, 681–689.
Stoerner H, Seith B, Hanemann U, George E, Rennenberg H. 1997. Nitrogen distribution in young Norway spruce (Picea abies) trees as affected by pedospheric nitrogen supply. Physiologia Plantarum 101, 764–769.
Strehler BL, Totter JR. 1952. Firefly luminescence in the study of energy transfer mechanisms. l. Substrate and enzyme determination. Archives of Biochemistry and Biophysics 40, 28–41.[Web of Science][Medline]
Tagliavini M, Millard P, Quartieri M, Marangoni B. 1999. Timing of nitrogen uptake affects winter storage and spring remobilization of nitrogen in nectarine (Prunus persica var. Nectarina) trees. Plant and Soil 71, 401–413.
Thompson JF. 1980. Arginine synthesis, proline synthesis, and related processes. In: Stumpf PK, Conn EE, eds. The biochemistry of plants, Vol. 5. New York: Academic Press, 375–402.
Toselli M, Flore JA, Marangoni B, Masia A. 1999. Effects of root-zone temperature on nitrogen accumulation by non-bearing apple trees. Journal of Horticultural Science and Biotechnology 74, 118–124.
Tromp J. 1983. Nutrient reserves in roots of fruit trees, in particular carbohydrates and nitrogen. Plant and Soil 71, 401–413.[Web of Science]
Tromp J, Ovaa JC. 1969. The effect of nitrogen application on the seasonal variations in the amino acid composition of xylem saps of apple. Zeitschrift für Pflanzenernahrung und Bodenkunde 60, 232–241.
Tromp J, Ovaa JC. 1976. Effect of time of nitrogen application on amino-nitrogen composition of roots and xylem sap of apple. Physiologia Plantarum 37, 29–34.
Wan X, Landhäusser SM, Zwiazek JJ, Lieffers VJ. 1999. Root water flow and growth of aspen (Populus tremuloides) at low root temperatures. Tree Physiology 19, 879–884.
Weinbaum SA, Klein I, Broadbent FE, Micke WC, Muraoka TT. 1984. Effects of time of nitrogen application and soil texture on the availability of isotopically labelled fertilizer nitrogen to reproductive and vegetative growth of mature almond trees. Journal of the American Society of Horticultural Science 109, 339–343.
Youssefi F, Brown PH, Weinbaum SA. 2000. Relationship between tree nitrogen status, xylem and phloem sap amino acid concentrations, and apparent soil nitrogen uptake by almond trees (Prunus dulcis). Journal of Horticultural Science and Biotechnology 75, 62–68.
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  S. Guak, D. Neilsen, P. Millard, R. Wendler, and G. H. Neilsen Determining the role of N remobilization for growth of apple (Malus domestica Borkh.) trees by measuring xylem-sap N flux J. Exp. Bot., September 1, 2003; 54(390): 2121 - 2131. [Abstract] [Full Text] [PDF]
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 E. Frak, P. Millard, X. Le Roux, S. Guillaumie, and R. Wendler Coupling Sap Flow Velocity and Amino Acid Concentrations as an Alternative Method to 15N Labeling for Quantifying Nitrogen Remobilization by Walnut Trees Plant Physiology, October 1, 2002; 130(2): 1043 - 1053. [Abstract] [Full Text] [PDF]
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